A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon water).
In lithographic processes, it is desirable frequently to make measurements of the structures created, e.g., for process control and verification. Various tools for making such measurements are known, including scanning electron microscopes (SEM), which are often used to measure critical dimension (CD). Other specialized tools are used to measure parameters related to asymmetry. One of these parameters is overlay, the accuracy of alignment of two layers in a device. Recently, various forms of scatterometer have been developed for use in the lithographic field. These devices direct a beam of radiation onto a target and measure one or more properties of the scattered radiation—e.g., intensity at a single angle of reflection as a function of wavelength; intensity at one or more wavelengths as a function of reflected angle; or polarization as a function of reflected angle to obtain a “spectrum” from which a property of interest of the target can be determined. Determination of the property of interest may be performed by various techniques: e.g., reconstruction of the target structure by iterative approaches such as rigorous coupled wave analysis or finite element methods; library searches; and principal component analysis. Compared with SEM techniques, optical scatterometers can be used with much higher throughput, on a large proportion or even all of the product units.
As technology develops, however, performance specifications become ever tighter. A further limitation of current methods is that they are made with optical wavelengths, requiring dedicated metrology structures with dimensions much greater than the typical dimensions of real product features. Accordingly, the measurements made on these metrology structures are only indirectly indicative of the real product structures. A particular parameter of interest is linewidth (CD), and a suitable small-target method for CD measurement has not yet been devised.
To obtain higher resolution measurements, it has also been considered to use EUV radiation, with wavelengths in the range for example 0.1 to 125 nm. EUV radiation is particularly attractive as it has wavelengths of the same order as the structures to be measured. Spectroscopic EUV reflectometry, for example, is proposed in the European patent application number 15160786, not published at the present priority date. Unfortunately, due to limitations of radiation sources available, there is no existing technique that provides for metrology on small targets, such as an in-die grating, or product structures themselves with a speed suitable for mass measurement in high-volume manufacturing. An ideal radiation source would be compact and affordable, and have a high brightness coupled with free selection of wavelength and good ability to focus into a small target area.
A bright, compact x-ray source has recently been described based on the phenomenon of inverse Compton scattering (ICS). This is described by W S Graves et al, in “Compact x-ray source based on burst-mode inverse Compton scattering at 100 kHz”, Physical Review Special Topics—Accelerators and Beams 17, 120701 (2014). The contents of the Graves et al reference and associated patent application are incorporated herein by reference. To reach high brightness on the electrons, a linear accelerator is used to the desirable high brightness for use in x-ray metrology applications. Details of a linear accelerator used in the x-ray source are provided in published patent application US2014191654A1 (Tantawi & Neilson). Other ways to accelerate the electrons are in development by other workers.